This invention relates to a liquid crystal display device, the liquid crystal display device is driven by a low voltage, a power consumption thereof is very small, a whole shape is made thin and small, and it is used for a watch, calculator etc. Recently, CMOS-LSI of semiconductor is developed, and a great dimension and capacity of liquid crystal display device are developed. Accordingly, a liquid crystal display device is applied to a personal computer and OA instruments. A liquid crystal display device will be used for many kinds of information processing instruments due to a merit of being directly driven with CMOS-IC. In this case, it is a very important problem how the liquid crystal display device attains a display capacity and response time as the same level of CRT display device. The present invention provides a liquid crystal display device which is able to have a great capacity and high speed response for high level information processing instruments.
In the conventional liquid crystal for a display use, a thermotropic liquid crystal is used and has many kinds of liquid crystal phase in different certain temperature ranges determined by a material thereof. As a rough classification, there are a nematic phase which has not a layer structure (referred to as N) and a smectic phase which has a layer structure (referred to as S m).
S m is classified as a smectic A phase of a axial characteristic (referred to as S m A) and a smectic C phase of a bi-axial characteristic (referred to as S m C). The thickness of a layer is almost equal to the length of one liquid crystal molecule.
FIGS. 1(a), 1(b) and 1(c) show various molecular alignments, FIG. 1(a) shows N, FIG. 1(b) shows S m A, and FIG. 1(c) shows S m C.
Furthermore, if a liquid crystal molecule has a asymmetric carbon atom and has not a racemic modification, it is aligned to form a spiral molecular alignment. In case of N, along axis of a liquid crystal molecule is located along a thin layer, and molecules align in one direction. A molecular direction in a layer is gradually twisted between layers to form a chiral nematic phase. FIG. 2 shows a molecular alignment the chiral nematic. In case of S m, a molecule is aligned in a spiral alignment in which a spiral axis alignes in a normal line direction of a layer to form a chiral smectic c phase (referred to S m C*).
FIG. 3(a) shows a molecular alignment of S m C*.
Referred to S m C* particularly, a long axis direction (referred to as a molecular axis hereafter) of a liquid crystal molecule in one layer is inclined by only angle .theta. relative to normal direction of a layer, and this angle .theta. is constant in every layers.
FIG. 3(b) shows the relation of a molecular axis and the normal direction.
On the other hand, in case when a molecular alignment of S m C* is viewed from the normal direction of layer, a direction angle .phi. successively rotates through layers according to a constant value (FIG. 3a shows a change by every 45.degree.) whereby a molecular alignment causes a spiral construction. Generally S M C* has not only a spiral construction but also an electric dipole in a vertical direction relative to a molecular axis and shows a ferroelectric characteristic.
A ferro-electric liquid crystal is discovered by Meyer in 1975 (J. de. phys. 36, 69, 1975). A synthesized liquid crystal is, generally speaking, DOBAMBC (2-methyl butyl P-[(P-n-decyloxybenzyliden)amino]) ##STR1## and it is used for a research of ferro-electric liquid crystal.
S m C* has a spiral construction, the pitch of the spiral construction differs according to a liquid crystal, and generally, it is several .mu.m as many cases. If S m C* liquid crystal is poured into a gap of a cell of 1 .mu.m which is thinner than the pitch of spiral, then a spiral construction disappears. A molecular alignment construction after a spiral construction disappeared is shown in FIG. 4 with a geometrical relation to a cell base plate. A liquid crystal molecule is aligned in parallel to the cell base plate, i.e., a molecular axis is in parallel with the base plate, and the liquid crystal molecule is aligned in an inclined condition of .theta. from the normal line direction of layer, and in this case, the normal line direction is parallel to the base plate.
Therefore, the layer is formed vertically relative to the base plate. In case of the inclined condition of .theta. from the normal direction of layer, there are domains in which a molecule is inclined +.theta. toward the clockwise direction from the normal line and other domains in which a molecule is inclined -.theta. toward the counterclockwise direction from the normal line.
S m C* liquid crystal molecule has generally an electric dipole vertical to the molecular axis. If the electric dipole is aligned toward an upper direction relative to the cell base plate in one domain, another electric dipole of another molecule is aligned toward a lower direction in another domain. If an electric field is applied between cell base plates, the whole liquid crystal molecules of the cell are aligned in an inclined position of +.theta. or -.theta. from the normal line direction of the layer (+ or - is determined by a direction of the electric dipole, and these are so called as +.theta. position or one of the bi-stable alignments and -.theta. position) or the other of the bi-stable alignments.
The liquid crystal molecule moves from +.theta. position to -.theta. position or -.theta. position to +.theta. position when the electric field is applied thereto in a direction opposite to the direction of the electric dipole. This phase construction is that of S m C since whole molecules of the cell are aligned in +.theta. position or -.theta. position, therefore, this bi-stable phase is made by the extinguishment of the spiral construction according to a thin gap of a cell. But, in the S m C, the molecule moves along a cone as shown in FIG. 3b as an image of spiral construction when it moves from .+-..theta. position to a contrary position thereof. An usual phase having a spiral construction does not cause this movement when an electric field is applied thereto. It is able to use the cell as a display device by moving or switching the liquid crystal molecule between +.theta. position and -.theta. position by attaching a polarizing member on a pair of cell base and by selecting electric field polarity.
FIGS. 5(a) and 5(b) show a relation between a pair of polarizing members and .+-..theta. position of liquid crystal molecule for use as a display device.
In FIG. 5(a), a polarizing axis of a polarizing member disposed on an incidence side corresponds to +.theta. position, a polarizing axis of another polarizing member disposed on an outgoing side is rotated by 90.degree. from the polarizing axis of the polarizing member on the incidence side.
In FIG. 5(a), a light which is polarized by the polarizing member on the incidence side is transmitted to the outgoing side without change of the polarizing direction when the liquid crystal molecules take +.theta. position, and the light does not pass through the outgoing side since the polarizing members cross with each other. This condition is a dark condition, or one of the two bi-stable display states. On the other hand, the polarizing direction of the light by a double refraction of the liquid crystal when the liquid crystal molecules is moved to -.theta. position. Said .theta. is 22.5.degree., and if the cell thickness is a preferable value, almost of the light passes through the polarizing member on the outgoing side whereby a bright condition or the other of the two bi-stable display states is obtained.
Therefore, it is necessary to have a relation between the cell thickness "d" and an anisotropy .DELTA.n of the refraction rate of the liquid crystal as follows: EQU d=(2n-1).alpha./.DELTA.n
n: refraction rate EQU .alpha.=c.pi./.omega. PA1 c: light speed PA1 .omega.: angular frequency of light PA1 (1) high speed response in .mu.sec order PA1 (2) memory characteristic PA1 (3) preferable threshold value characteristic PA1 V.sub.th =500 (mV) PA1 V.sub.sat =5 (V) (measured by DOBAMBC)
FIG. 5(b) shows a condition in which the two polarizing members on the incidence and outgoing sides are same so that +.theta. position is bright and -.theta. position is dark. A relation of the cell thickness and .DELTA.n is the same as above formula, preferable bright and dark conditions are attained when .theta.=22.5.degree.. This idea of display device is presented by Clark and Lgerwall (Appl. phys. lett. 36, 899, 1980). They insisted that a display device in which a thin cell has a pair of polarizing members has a feature as follows:
The high speed response among these characteristics is confirmed by our experimentation.
Further, the memory characteristic namely the stability of the bi-stable alignment, which is able to keep the +.theta. positions without application of the electric field after one of .+-..theta. positions was set by applying an electric field thereto, is also confirmed. But, a preferable threshold value characteristic is not confirmed by us.
FIG. 6 shows a relation between an optical transparent intensity I of the display state and an applied voltage V when there is a threshold value characteristic.
A molecule does not move and the optical transparent intensity does not change when the applied voltage is less than V.sub.th. The molecule begins to move at more than V.sub.th and, at this time, the optical transparent intensity remarkably changes according to the applied voltage.
If the applied voltage becomes greater than V.sub.sat, the optical transparent intensity will not change any more, because the molecule is fixed in .+-..theta. position. These V.sub.th and V.sub.sat are good parameters to represent the threshold value.
According to our data, V.sub.th and V.sub.sat are as follows:
It is not able to drive a display device by 5V in a selected point and 500 mV in a non-selected and a half selected point during a time-sharing drive of the display.